Sequential equivalent-time sampling oscilloscopes such as the DSA8200 Digital Serial Analyzer available from Tektronix, Inc. of Beaverton, Oreg. and the 86100C Infiniium DCA-J available from Agilent, Inc. of Santa Clara, Calif. are used to measure time-domain waveforms of high-frequency signals. These oscilloscopes consist of a mainframe into which various models of sampling modules are installed.
The sampling circuits used in the sampling modules have high instantaneous measurement bandwidths but they must sample at low sampling frequencies. For example, a Tektronix 80E06 sampling module provides 70 GHz of bandwidth but the DSA8200's maximum sampling rate is 200 kHz. Similarly, an Agilent 8611B provides 65 GHz of bandwidth but the 86100C's maximum sampling rate is 40 kHz. Accordingly, in typical usage the waveforms acquired by these oscilloscopes are aliased.
To acquire one sample, the oscilloscope arms a trigger circuit and waits for an edge of a user-supplied trigger signal. Once a trigger edge is detected, the oscilloscope creates a precision time delay by running a delay generation circuit which consists of a startable oscillator, a counter, and a vernier. After the precision time delay elapses, the oscilloscope strobes each sampling circuit installed into the mainframe and in response each sampling circuit produces a sampled analog value. The sampled analog values are digitized by analog-to-digital converters and then the digitized values are passed to a controller for processing, storage, and display. The oscilloscope then waits up to 1/(200 kHz)=5 μs or 1/(40 kHz)=25 μs as the case may be to provide time to process the samples and prepare for the next acquisition. The oscilloscope then re-arms the trigger circuit and waits for the next trigger edge.
To acquire a complete waveform, the oscilloscope performs the above steps multiple times as the delay generation circuit is sequentially stepped through a range of delays. For example, to acquire 1,000 samples covering a 1 ns time-span, the sample spacing thus being 1 ps, the oscilloscope performs the above steps 1,000 times as the delay generation circuit is stepped through a 1 ns time span in 1 ps increments. The resulting samples represent the signal-under-test at “equivalent times” of 1 ps, 2 ps, 3 ps . . . 1,000 ps relative to the trigger signal. If the signal-under-test is repetitive and the trigger signal is synchronous with the signal-under-test, then the waveform appears stationary on the display-an effect referred to as “the stroboscopic effect.”
One difficulty with these oscilloscopes is that propagation delay imperfections in the trigger circuit, the delay generation circuit, and the sampling circuit may cause the oscilloscope to acquire samples slightly earlier or later than desired, thus giving the appearance of jitter in the displayed waveform and spoiling the waveform fidelity. See J. B. Rettig and L. Dobos, “Picosecond time interval measurements,” IEEE Transactions on Instrumentation and Measurement, vol. 44, no. 2, pp. 284-287, April, 1995 for a detailed discussion of oscilloscope jitter. Both the DSA8200 and the 86100C have approximately 700 fs RMS of intrinsic random Gaussian jitter. Sampling modules such as the Tektronix 82A04 Phase Reference Module and the Agilent 86107A Precision Timebase Reference Module provide for the post-processing compensation of oscilloscope jitter by simultaneously sampling quadratures of a reference clock that is synchronous with the signal-under-test, transforming those quadrature samples into phase values, determining the timing error of the oscilloscope from those phase values, and then adjusting the horizontal placement of the samples of the signal-under-test to compensate for that timing error. See U.S. Pat. No. 6,564,160 to Jungerman et al. for a discussion of the Agilent 86107A. However, in many cases a synchronous reference clock is not available and thus this approach cannot be used.
Another difficulty with these oscilloscopes is that the oscilloscope cannot provide information about the “absolute” jitter of the signal-under-test, that is, jitter relative to a spectrally pure time reference. In other words, if both the trigger signal and the signal-under-test jitter in the same fashion, then the oscilloscope display nonetheless appears stationary and gives no indication of that jitter to the user because the oscilloscope only acquires samples at times relative the trigger signal. Knowledge of the absolute jitter of the signal-under-test is useful because it is specified in many serial data standards.
What is desired is a sequential equivalent-time oscilloscope having reduced jitter without the use of a reference clock that is synchronous to a signal-under-test. It is further desired that the oscilloscope indicate the absolute jitter of the signal-under-test.